Bodies are very intelligent. As soon as you put one bite of a food in your mouth, your body knows whether it will be able to digest that food or not. And it acts accordingly. If you can digest that food, your body will start getting ready to receive and work with that food by creating the needed enzymes down through the intestinal tract. Now, you really can't tell if this positive action is happening or not... But, if your body cannot digest that bite of food, a whole different set of reactions gets triggered. The body immediately will go into a defense or immune reaction when it realizes you have put something in it that it can't handle, that it is allergic to or that it can't create the enzymes needed to digest it. And at least one of these reactions is very easy to see.